The site said that Breitbart died of "natural causes" shortly after midnight on Thursday. ABC News confirmed that Breitbart had passed away. Breitbart's attorney also confirmed the news to CNN.

Breitbart's father-in-law told the Associated Press that Breitbart was walking near his house when he collapsed. Someone saw him fall and called 911. Breitbart was taken to UCLA Medical Center, but could not be revived.

"We have lost a husband, a father, a son, a brother, a dear friend, a patriot and a happy warrior," the post said. "Andrew lived boldly, so that we more timid souls would dare to live freely and fully, and fight for the fragile liberty he showed us how to love."

Breitbart came to be well-known for his work with the Drudge Report (he also played an early role with The Huffington Post), and would go on to found the Big Journalism, Big Hollywood and Breitbart.com websites. He was also an author, columnist and ubiquitous commentator in the media.

Groups from all sides of the political spectrum paid tribute to Breitbart. Republican gay rights group GOProud, of which Breitbart was a board member until recently, released a statement saying it was "stunned and saddened," and calling him an "amazing friend and ally."

Liberal media watchdog Media Matters also issued a statement. "We've disagreed more than we've found common ground, but there was never any question of Andrew's passion for and commitment to what he believed," the group said.

Arianna Huffington, the editor-in-chief of The Huffington Post Media Group, also paid tribute:

I was asked many times this morning for my thoughts on what Andrew meant to the political world, but all I can think of at the moment is what Andrew meant to me as a friend, starting from when we worked together -- his passion, his exuberance, his fearlessness. And above all, what I'm thinking of at the moment is his amazing wife Susie and their four beautiful young children. My love and thoughts are with them right now.